In recent years, demands for down-sizing and thickness reduction of semiconductor devices for communications have been increasing in the field of mobile communications with a focus on mobile phones. In mobile phones, with the improvement of functions, space for mounting semiconductor devices for communications has been narrowed. Thus, in place of a configuration in which packaged semiconductor devices are mounted on a substrate, the number of cases where a bare chip having protruded electrodes (hereafter referred to as “solder bumps”) is directly mounted on a substrate has been increased.
Furthermore, reducing the size of a semiconductor device with semiconductor elements mounted thereon to be smaller than 0603 size (0.6 mm length×0.3 mm width×0.3 mm height), which is the size of a chip-type passive part, has been studied.
In such a semiconductor device having a height of not more than 0.3 mm, heights of solder bumps often become not more than 100 μm, i.e. micro bumps in many cases, and as a result of using bare chips, the pitch between the bumps becomes nearly 200 μm and the bumps mutually come close. Furthermore, since a receiving land provided on a mounting board is 100 μm or less square, solder printing on the receiving land of the mounting board is difficult.
Since the diameter of the micro bump is small, the quantity of solder decreases, and since the bump pitch between the micro bumps is narrowed, the area of the receiving land of the mounting board is reduced. For these reasons, when a bare chip having micro bumps is mounted on a board, soldering mounting is often used in which only flux is applied to the mounting board to mount the bare chip and reflow heating is performed.
At the same time, if the heights of solder bumps vary, disadvantageously, defective soldering is likely to occur. Specifically, if there is a portion where the solder bumps do not contact the receiving land of the mounting board due to the variation of the heights of the solder bumps, even if reflow heating is performed, heat is not sufficiently transferred to the solder bumps separated from the receiving land of the mounting board. Therefore, solder may not be melted to cause poor characteristics, and the shape of the solder may be deformed to cause poor appearance.
FIG. 12 is a sectional view showing the configuration of a conventional semiconductor device (bare chip). The semiconductor device shown in FIG. 12 is 4 mm square, and has the total thickness of 0.3 mm including the height of a solder bump. On the surface layer of a wafer of a substrate 51, 56 connecting pads 54 are formed in 0.25 mm pitch along the periphery of the chip. On the surface layer of the wafer of the substrate 51, a thin insulating protective film 55 is also formed so that the surfaces of the connecting pads 54 are exposed.
An underlying electrode 56 is formed on each of the connecting pads 54, and a solder bump 52 composed of SnAg having a height of 90 μm is formed by plating on each underlying electrode 56. In ordinary plating processes, after forming underlying electrodes 56, a plating resist is applied, openings for the underlying electrodes are formed by exposure, and SnAg solder bumps 52 are formed by electrolytic plating. Then, after removing the plating resist film, reflow heating is performed to process the solder bumps 52 to be spherical.
However, since solder bumps are formed by a plating process, the above-described semiconductor device has a problem of the variation in the height of bumps, and the variation in the height of the completed bumps tends to be large as the plating resist film is thicker. For example, when the target value of the height of the completed bumps is 30 μm, the tolerance becomes about 3 μm; when the target value of the height of the completed bumps is 50 μm, the tolerance becomes about 10 μm; and when the target value of the height of the completed bumps is 90 μm, the tolerance becomes about 15 μm; and the tolerance increases with an increase in the target value.
When such semiconductor devices are mounted on the receiving land of the mounting board, the solder bumps support the semiconductor devices on the mounting board. Therefore, the distance between the substrate of the semiconductor device and the mounting board is determined at a portion of the solder bump having a large height, and the solder bumps having a small height are in a state where the solder bumps do not reach the receiving land of the mounting board and are isolated from the receiving land. Such a state is easily produced as the target value of the height of completed bumps is larger, and further the number of solder bumps increases.
With solder bumps having a height of not more than 100 μm, the receiving land of the mounting board is too small to perform solder printing. Therefore, since heat is not transferred if reflow heating is performed in the state where the solder bumps are isolated from the receiving land, disadvantageously, solder is not melted to cause defective connection, and only a part of solder bumps are melted to make the bump shape abnormal.
It is an object of the present invention to provide a semiconductor device and a method of manufacturing the same that can realize the stable yield of solder connection even if the height of solder bumps varies, with the configuration of the solder bumps capable of absorbing the variation of the height of the bumps during reflowing in mounting the semiconductor device on a mounting board.